1. Field of the Invention
This invention relates generally to a dual tensioner for tensioning a power transmission belt of a belt drive system. Particularly, it relates to a dual tensioner having a linear pulley movement. Specifically, this invention relates to a linear movement tensioner having two tensioner pulleys whose movement is mechanically damped with included applicability to power transmission belt drive systems having a unitary device performing both the engine starting function and the electrical power generation function, such as a motor/generator sometimes referred to as a Gen-Star.
2. Description of the Prior Art
Internal combustion engines, or power plants, commonly use power transmission belt drive systems to tap power from the engine's crankshaft and deliver it to one or more various engine auxiliaries or accessories. In automotive applications, these accessories include power steering pumps, water pumps, air conditioning compressors, fuel pumps, and alternators. Historically, such engines have had the main power takeoff point at the crankshaft protruding from the rear of the engine to which is attached the drive train for driving the wheels to move the automobile. The accessories are driven from a pulley attached to the front of the crankshaft. Each accessory is equipped with a pulley. All of the pulleys are in mechanical communication via one or more power transmission belts trained about them. Some method of tensioning each power transmission belt is provided. The power transmission belt, the pulleys, and devices accomplishing belt tensioning form the accessory belt drive system.
Earlier systems included multiple v-belts. Commonly, each belt was tensioned by manual adjustment and fixing of the position of one accessory or idler per belt. These are referred to as locked-center belt drives, because there is no provision for automatic movement of any of the pulleys to accommodate varying condition of the belt or of the drive as a whole. If the belt should stretch or otherwise lengthen, the tension upon the belt would lessen. Further, for proper operation of the belt drive system, the tension of the belt must be set high enough to accommodate the worst case condition. Such worst case conditions can be the result of extremes of temperature, engine operation, or accessory operation.
There has been interest in making the volume, of the engine compartments of automobiles, smaller. To accommodate the smaller compartments, various aspects of the engines have become smaller, including the accessory belt drive systems. This has been accomplished, at least in part, by reducing the number of belts employed. As each belt is removed, and the number of layers extending from the front of the engine is thereby removed, the total distance the belt drive system extends from the front of the engine is reduced. Ultimately, this has resulted in the use of a single serpentine belt for many applications. A serpentine belt is so named because of the way it snakes around the various pulleys in a series of bends, both forward and backward. A v-ribbed or Micro-V (a registered trademark of The Gates Rubber Company) belt is most suited to serpentine applications.
The limitations of the locked-center approach to belt tensioning are exacerbated in serpentine applications. Accordingly, most modem serpentine belt drives include an automatic tensioner whereby the changing conditions of the belt drive system can be better accommodated. In basic form, an automatic tensioner has a framework or attachment point, which attaches directly to the cylinder block of the engine, or indirectly to some point on the vehicle that is stationary with respect to the engine of the vehicle, and a pulley, which presses upon the belt in the plane of rotation of the belt drive system. A moveable member or connective portion extends between the framework and the pulley to provide pressure upon the belt, via the pulley. The pressure acts to lengthen the distance about which the belt is trained and thereby causes the belt to be in tension. Various techniques and geometries have been employed to provide the biasing force. Commonly, a resilient member, such as a steel spring acts to force the moveable member in rotating motion which results in the pulley tending to move in a direction toward a surface of the belt which, in turn, tends to increase tension upon the belt.
A tensioner with only these elements provides a somewhat constant force upon the surface of the belt when the system is in a resting state (i.e., the pulleys are not rotating). Dimensional instability, of the drive system caused by time, temperature, or manufacturing variation is accommodated fairly well through the action of the resilient member, at least to the limits of the resilient member and geometry of the tensioner. Thus, the tension upon the belt remains relatively constant, when the system is at rest, even though the belt may have stretched or the engine may be hot or cold. However, a tensioner with only these elements may not maintain appropriate tension upon the belt for all operating conditions of the system.
An operating power transmission belt drive system typically oscillates due to the influences of torsional vibration or other angular acceleration of the crankshaft or accessories, the influences of unbalanced conditions, or other influences. Torsional vibration of the crankshaft occurs, in part, as a result of the distinct impulses delivered to the crankshaft through the combustion cycles of each cylinder and piston combination. The oscillations lead to vibration of the belt. This, in turn, leads to vibration of the moveable portions of the tensioner. Momentum then builds in those moveable portions modifying the force the pulley exerts upon the belt surface and the tension upon the belt. The changing tension upon the belt can cause unacceptable performance for the belt drive system. In one instance, issues of short-term performance, such as where the belt of the belt drive system slips excessively limiting the system's efficiency or power transmission capability, or is excessively noisy due to slippage or otherwise, can arise. In another instance, the amount of tension necessarily applied to the belt, to have acceptable performance on the short-term, leads to long-term issues such as premature failure of one or more components of the system, including the belt, or one or more accessories.
To accommodate these issues and thus improve the performance of tensioners, damping devices have been included in tensioners. Early damped tensioners have included symmetrical damping where movement of the moveable portions of the tensioners are damped approximately equally whether the instantaneous movement is in the direction tending to increase tension upon the belt or in the direction tending to decrease tension upon the belt. Damping combines with the forces supplied by the resilient member to result in a modified biasing, at the pulley/belt interface.
Other tensioners have utilized asymmetrical damping. Commonly, such tensioners are damped such that the damping upon the moveable portion is minimal when the tensioner is moving in the belt tensioning direction and maximal when moving in the belt loosening direction. In one approach, a shoe is biased against a race at an angle different from normal to the surface of the race. As a result, the relative movement of the shoe and race in one direction tends to lift the shoe from the race. This reduces the pressure at their interface, reduces the friction that gives rise to the damping, and thereby reduces the damping. The other direction tends to wedge the shoe against the race and increase the damping. An example is described in U.S. Pat. No. 5,964,674, to Serkh et al. These have involved the use of tensioners having a single pulley biased against a surface of the belt, to supply tension. Further, the biasing against the belt has been solely relative to the engine block.
U.S. Pat. No. 4,416,647 to White, Jr. discloses the use of tensioners with two pulleys pressing upon the power transmission belt. The '647 patent states that the approach is useful for tensioning a system with a cyclic load such as an air conditioning compressor. One, of the pulleys, presses upon a span of the power transmission belt immediately upstream of the cyclic load. While, the other pulley presses upon the power transmission belt immediately downstream of the cyclic load. In one embodiment, the two pulleys are fixed relative to each other upon a angled member that can pivot about its apex. The assembly is pressed toward the power transmission belt to supply static tension in a locked-center fashion. The pivot is said to accommodate dynamic tension. Static tension is the result of the force applied to the power transmission belt by the tensioner in the belt tensioning direction with the effect of tending to lengthen the distance the power transmission belt is forced to travel about the pulleys of the system. If it were assumed that each of the pulleys of the system is allowed to rotate freely, tension on every span would be the same and at static tension. Dynamic tension is the tension over the length of the power transmission belt that is the result of static tension as altered by the influences of torque upon each of the pulleys and various imbalances of the system. As an additional result, each span tends to be under differing tension.
In another embodiment, each of the two pulleys is affixed to a separate arm that can move about the pivot, individually. The two arms are biases toward each other by a spring. The '647 patent indicates that either embodiment is damped by the interaction of the pulleys with the separate power transmission belt spans. There is no indication that friction or other damping is introduced at the pivot, whether movement of the pulleys is in relation to the engine or to each other.
Power transmission belt tensioners used in conjunction with accessory belt drive systems for internal combustion engines are known with pulley movements that are both arcuate and linear. Those of the above discussion have all used arcuate movements. Certain applications benefit from linear movement tensioners (i.e., those where the idler pulley contacting the power transmission belt moves in a substantially straight path). An example of a linear movement tensioner can be found in U.S. Pat. No. 4,634,408 to Foster. The '408 Patent discloses the construction of a relatively complex tensioner that incorporates a plurality of springs about a hydraulic damping mechanism. Further, the carrier, upon which the tensioning pulley is mounted, engages the track, that defines the carriers two degrees of freedom of movement, with a ball and track bearing structure. The inclusion of hydraulics creates the concern of leaking hydraulic fluid. The hydraulic mechanism disclosed is apparently not compact. Thus, the tensioner appears to be relatively large. The relatively large number of parts adds to cost and complexity of construction. Further, there is no provision for compensating for the parasitic torque across the carrier that supports the idler pulley, that tends to twist that carrier, other than the effectiveness of the ball and track bearing structure.
Traditionally, an electric starter motor is provided to spin the crankshaft of the engine so that combustion may be initiated and the engine will begin to run. The starter motor is located near the rear of the engine and is adapted to intermittently engage the rear portion of the crankshaft through a gear train.
Currently, there is increasing pressure to reduce emissions and increase fuel economy by lowering the weight of the automobile and reducing the number of under-the-hood components. An approach taken toward these goals involves combining the function of the starter motor and the function of the alternator into a single device, a motor/generator or a Gen-Star. Also toward the goal of increasing fuel economy, the Gen-Star promotes the use of a feature called “stop-in-idle”. This feature is where the engine is allowed to die when it would ordinarily idle, then be restarted when the automobile is expected to resume motion. This feature substantially increases the demands placed upon accessory belt drives. In application, the motor/generator is placed in mechanical communication with the crankshaft via the accessory belt drive. The motor/generator and associated accessory belt drive system tends to be placed at the front of the engine. However, placing these systems at other locations, including the rear of the engine is envisioned.
The advent of Gen-Star systems causes the designer, of power transmission belt drive systems, to face substantial new challenges, above mere oscillatory loads. A significant challenge, among these, has been to develop a tensioning system that results in acceptable performance, by an accessory belt drive that includes this new device, which not only offers substantial load and rotational inertia, but also adds large driving torque into the accessory belt drive. Further, it provides this large driving torque on an intermittent basis.
U.S. Pat. No. 4,758,208 to Bartos et al., discloses the use of two arms with arcuate movements, each carrying a pulley. The arms are mounted with pivot points that correspond with the shaft of a Gen-Star. The two arms are biased toward each other by a spring. The tensioner also includes having the Gen-Star mounted in a limited rotatable fashion such that the housing is allowed to rotate a few degrees in reaction to whether the Gen-Star mode is of operating as a starter or an alternator. This reactive movement operates a pair of latches, which alternatingly lock one or the other of the two arms against movement, depending upon mode. In this manner, the arm associated with the power transmission belt span terminating at the Gen-Star pulley with the greatest tension, as a result of the Gen-Star mode, is locked in place. The free arm then supplies tension to the power transmission belt system. This tensioner is apparently complex, requires special mounting of the Gen-Start having moving parts subject to wear, and is not flexible in its application. Further, the '208 patent does not disclose the contemplation of adding damping to the movement of either pulley to enhance system performance.
Accordingly, there is a continuing need for a linear movement tensioner that is simple, compact, durable and devoid of hydraulics and provides the benefit of two pulley operation for those applications that can benefit therefrom. An accessory power transmission belt drive system having a Gen-Star is such an application.
Further, there remains the need for a tensioner and system, for use in conjunction with a Gen-Star that provides, at once, adequate short-term performance, adequate long-term performance, optimizes the width of the belt that may be used for any given application, contains cost and complexity, is flexible according to the Gen-Star systems to which it can be applied and offers a linear movement.